Spark anodizing of β-Ti alloy for wear-resistant coating

Spark anodizing of a bcc solid solution Ti–15% V–3% Al–3% Cr–3% Sn alloy has been performed in an alkaline electrolyte containing aluminate and phosphate using dc-biased ac anodizing to form a wear-resistant coating on the alloy. The coating consists mainly of Al2TiO5, with rutile and γ-Al2O3 being present as minor oxide phases. Depth profiles of the coating, examined by glow discharge optical emission spectroscopy, have revealed that aluminium species, highly enriched in the coating, distribute uniformly in the coating, while phosphorus species, incorporated from the electrolyte, are located mainly in the inner part of the coating near the coating/alloy interface. The location of the phosphorus species should be associated with the porous nature of the coating, allowing access of the electrolyte directly to the inner parts of the coating. The porosity of the coating is reduced by anodizing to high voltages. The marked improvement of the wear resistance by the coating has been demonstrated from a pin-on-disc wear test.